The invention relates primarily to a threaded fastening member including an actuating end having a head, the head having a diameter projecting beyond a diameter of the thread of the fastening member and being coaxial with a thread axis; and a centrally located recess within the head for the application of interior forces.
The recesses of these types of prior art screws which are intended for the application of force from the interior, are configured symmetrically. Their axis of symmetry is the axis of the screw thread. The recesses of known screws of this type have different shapes. They may be cruciform, hexagonal, square, rectangular or circular with notched toothing, wedge-shaped toothing or arcuate toothing.
It is known that significant improvements in securing screws against loosening and with respect to their fatigue strength are obtained if they are pre-tensioned to the highest possible level of their pre-tensioning force, i.e., approaching the force at which their shank would break. This high pre-tensioning force cannot be realized with many of the known types of force application to a screw, because the tightening torque is limited. This torque limitation, for example, has the following mechanical causes: reaming of the interior-force application zone, severing of the head due to insufficient strength of the material between the head and the transition to the shank and the base of the interior-force application zone, and breakage of the screwing tool.
One important cause for the limit of the pre-tensioning force of conventional screws of this type is also that the screwing tool, due to axial forces during the assembly process, is pushed axially out of its force application zone. This cam-out effect is the result of a succession of reactive forces between the screwing tool and the force application zone, i.e., in the last analysis, it is the result of the screw torque that is applied.
A further drawback of most configurations of interior-force application zones for a screw is that different tools are required for screwing in different nominal diameters. For example, for mounting many different types of conventional Phillips screws having a nominal diameter ranging from 1.6 to 10 mm, five different sizes of screwdrivers are required. Frequently, each size of screw requires a screwdriver whose size specifically matches the screw.